The main features of the complete mix or CSTR design include the tanks, mixers, covers and heating systems. Some CSTR designs use a single mix tank where all phases of treatment (hydrolysis, methanisation etc) happen in a single tank. This tank would sometimes have a flexible membrane cover to store resulting biogas and would utilize mixers that mount on the side of the tank and penetrate into the tank generally with a propeller or mixing jets to mix the tank’s contents. Some CSTR systems will split treatment into two tanks running in series. This style accomplishes hydrolysis and acidification in the first tank and the methanisation in the second tank. This style is commonly noted by a first stage with a fixed roof with a roof mounted, high torque, slow speed propeller style mixer mounted from the top. The second stage methanisation tank will be noticeable by its domed shape flexible gas cover as shown in the picture below.
CSTR Mixing Systems
There are three chief means of mixing a CSTR digester; by gas, mechanical or hydraulic mixing techniques. Gas mixing systems operate by compressing biogas and injecting it into the tank contents. Resulting rising gas bubbles dislodge the contents either in the bottom or top of the digester and keep them stirred. There are two types of mechanical mixing systems, those with their moving parts in the basin and those with their moving parts outside the basin. The former commonly use either side mounted propeller style mixer and a hoist mechanism to facilitate easy motor maintenance or a roof mounted propeller. The latter will commonly employ mixing jets where tank contents are first pulled out of the basin by a motive pump and reintroduced at a high velocity back to the tank through a jet nozzle to keep contents stirred and agitated. Properly designed mixing systems will commonly “spin” the tanks contents on a vertical axis. Common design consideration and operational issues to watch are the potential for grit accumulation on the tank bottom and the possible accumulation of a floating fat or scum layer on the top of the tank. Well designed mixing systems will be able to keep bottom solids agitated and top floating layers “folded in” to the mixed digester contents. The photo below shows a propeller style mixer that penetrates the tank wall and is angles to encourage the tank contents to spin horizontally.
It is common to design a CSTR where the mixing system is not operated continuously. Many mixer suppliers will have their systems operate for just 30-70% of the running time. The anaerobic treatment process continues despite the fact that the contents are not actively being stirred constantly.
CSTR Tank Selection
CSTR systems have been installed using a variety of tank materials including both concrete and steel. Each style has proven to provide years of robust service. The most common CSTR configuration is a round tank. The roofs on CSTR tanks can either be fixed concrete, fixed steel, floating or a flexible membrane.
Steel tanks can be installed using either stainless steel or carbon steel. Internal coatings on the tanks include glass linings or painted epoxy coatings to prevent corrosion. Steel tanks incorporate panels that are either welded or bolted together. A typical steel digester tank with cladded rigid insulation attached to the exterior is shown below:
Great care should be taken to be sure that aggressive tank contents do not harm tank internals. Corrosive conditions are common inside digesters especially in the gas/ water head space intersection. Grit laden digester contents can abrade digester tanks and equipment. Any sealant, gasket or fastener used in tank construction needs to be properly evaluated to ensure long service life.
Concrete tanks can be implemented by either poured-in-place methods or by use of precast and pre-stressed panels. The same methods can be used on concrete tank roofs. Precast panels require proper care be taken to join the panels in a way that ensures gas tightness. Both concrete and steel tanks can be insulated for use colder climates from a wide variety of insulation types including spray-on or rigid insulation board.
Tanks are found that are both short and wide or taller and narrower in design. Either configuration can work well. For any given application there will be an optimum design that provides the best cost and the best functionality to maximize gas yield and minimize service and maintenance.
Digester Tank Covers
There are several styles of cover for a wet digester tank. Covers can be either fixed or floating. They can be made of a rigid material, like steel or concrete, or they can be made of a flexible material such as a canvas membrane. Flexible membrane covers generally have an inner cover that is able to flex as gas pressure changes and an outer membrane that is kept fully inflated by a small fan such as in the photograph below.
Tanks that have a rooftop mixer will require a rigid roof structure to support the mixer. See photo below for a typical digester steel roof using bolted stainless steel panels:
Digesters can also be implemented as earthen, lined lagoons. Lagoons have the benefit of being very inexpensive to install. They are more commonly applied in warmer climates as systems in colder climates require a more engineered approach to control temperature, mixing and gas collection. Lagoons are most commonly applied in animal manure projects or in wastewater treatment where waste volumes are large and land is plentiful.
Digesters need to be kept at a steady, warm temperature for optimum gas yield and stable system operation. Ideally digester heat requirements can be met by capturing waste heat from other facility unit operations, such as a Combined Heat & Power (CHP) system. Sometimes, such as during start up, supplemental heat from a gas boiler is required to get a system up to operating temperature. Heat can be applied internally to the tank through a series of hot water pipes that are either embedded in concrete tank walls or supported on the inside tank walls.
Heat can also be applied externally to the tank from an external heat exchanger where contents from the tank are heated in either a shell-in-tube or plate heat exchanger using a heat transfer fluid, usually water, that pulls heat from the CHP process that has waste heat and moves heat to the tank contents.
The ideal digester scenario is one that never needs to be taken out of service during their long, useful life. It is therefore important to be able to remove settled grit that can accumulate over time. Systems that employ a rotating scraper arm across the tank bottom, for one, can be used to force contents to the tank wall where they can be properly removed. Tanks can also be installed with sloped bottoms that force contents to the sidewall or tank middle for removal. The optimum pre-treatment system will keep as much grit as possible out of the tank ahead of time but there’s no way to keep it all out so it is important to remove grit to keep 100% of the tank’s usable digestion volume functioning. Unfortunately, applications where there are heavy loads of grit or inerts in the feedstock can require the tank be taken out of service every few years if the level of inerts are allowed to build up on the tank bottom.